From the viewpoint of global environment preservation, the automobile industry is recently oriented to improving the fuel efficiency of automobiles to meet CO2 emissions regulations. Because automobile fuel efficiency is enhanced most effectively by reducing the weight of automobiles through thinning of parts, high strength hot rolled steel sheets are increasingly used as automobile part materials. On the other hand, as many of automobile steel parts are produced by press forming, steel sheets for automobile parts are required to exhibit excellent press formability in addition to high strength.
However, increasing the strength of steel sheets tends to increase the anisotropy in mechanical properties. Consequently, steel sheets strengthened to a tensile strength of not less than 850 MPa have a marked anisotropy in mechanical properties. Due to this anisotropy, problems are encountered such as unexpected occurrence of cracks during press forming and a failure of the press-formed articles to attain desired properties such as impact resistance depending on the direction in which a press load has been applied.
For the reasons described above, application of high strength hot rolled steel sheets with a tensile strength of not less than 850 MPa to automobile parts and the like entails the development of a technique realizing stable industrial-scale production of high strength hot rolled steel sheets with a low anisotropy in mechanical properties.
A variety of techniques have been proposed with respect to high strength steel sheets for automobile parts.
For example, Japanese Unexamined Patent Application Publication No. 2011-026690 proposes a technique in which the chemical composition of a steel sheet includes, by mass %, C: 0.02 to 0.08%, Si: 0.01 to 1.50%, Mn: 0.1 to 1.5%, Ti: 0.03 to 0.06%, P: not more than 0.1%, S: not more than 0.005%, Al: not more than 0.5%, N: not more than 0.009%, and a total of Nb, Mo and V: not more than 0.01%, the balance being Fe and inevitable impurities, the ratio Ti/C of the Ti content to the C content being 0.375 to 1.6, and in which TiC precipitate in crystal grains has an average diameter of 0.8 to 3 nm and an average numerical density of not less than 1×1017 [particles/cm3]. According to the technique proposed in JP '690, titanium having the highest carbide-forming ability is effectively utilized in precipitation strengthening and consequently alloy-saving high strength hot rolled steel sheets are obtained which are prevented from a decrease in workability due to the addition of alloy elements and have a tensile strength of 540 to 650 MPa.
Japanese Unexamined Patent Application Publication No. 2007-302992 proposes a technique in which the chemical composition of the steel sheet includes, by mass %, C: 0.015 to 0.06%, Si: less than 0.5%, Mn: 0.1 to 2.5%, P≦0.10%, S≦0.01%, Al: 0.005 to 0.3%, N≦0.01%, Ti: 0.01 to 0.30% and B: 2 to 50 ppm, the balance being Fe and inevitable impurities, the atomic ratio of carbon to carbide-forming elements is controlled to a specific value, the contents of silicon, manganese, boron and molybdenum, which are elements controlling the γ/α transformation temperature of steel, are specified to satisfy a predetermined relationship, and the microstructure of a steel sheet is configured such that the area percentage of one or both of ferrite and bainitic ferrite is not less than 90%, and the area percentage of cementite is not more than 5%. According to the technique proposed in JP '992, addition of boron remedies a decrease in grain boundary strength caused by carbide precipitation to control the occurrence of defects at burrs. Consequently, it is allegedly possible to inexpensively and stably manufacture high strength hot rolled steel sheets with a tensile strength of not less than 690 MPa which exhibit excellent stretch flangeability as well as have high crack resistance in blanking and good surface condition.
Japanese Unexamined Patent Application Publication No. 2002-105595 proposes a technique in which the basic chemical composition of a steel sheet includes, by mass %, C: 0.01 to 0.10%, Si: not more than 1.0%, Mn: not more than 2.5%, P: not more than 0.08%, S: not more than 0.005%, Al: 0.015 to 0.050% and Ti: 0.10 to 0.30%, the balance being Fe, and the microstructure of the steel sheet is configured such that the main phase is ferrite, the unit grains are grains surrounded by adjacent grains each with an orientation difference of not less than 15°, and the average unit grain diameter thereof, d μm, is not more than 5 μm. Further, JP '595 proposes a method of manufacturing high strength hot rolled steel sheets in which steel having the above chemical composition is heated, rolled, cooled and coiled such that finish rolling is performed at a temperature of 900 to 840° C. and the reduction ratio in the finish rolling is not less than 70%. According to the techniques proposed in JP '595, high strength hot rolled steel sheets with excellent stretch flangeability are allegedly obtained by controlling the diameter and configuration of ferrite grains.
Japanese Unexamined Patent Application Publication No. 2-008349 proposes a technique in which the chemical composition of a hot rolled steel strip includes C: 0.04 to 0.18 wt %, Si: 0.05 to 1.00 wt %, Mn: 0.10 to 0.50 wt %, Ti: 0.05 to 0.30 wt %, Al: 0.001 to 0.100 wt %, N: not more than 0.0100 wt %, P: not more than 0.030 wt % and S: not more than 0.015 wt % and satisfies 0.3≦Ti/(C+S+N)<5 and C+Mn/6+Si/24+Cr/5≦0.20 wt %, and the polygonal ferrite fraction in the final microstructure is not less than 70%. According to the technique proposed in JP '349, the Si and Mn contents are controlled to reduce the carbon equivalent weight which is an indicator of weldability, and further the prescribed amount of titanium as a strengthening component is added, thereby obtaining high tensile strength hot rolled steel strips having excellent weldability and a tensile strength of not less than 55 kgf/mm2.
According to the technique proposed in JP '690, the Ti content is as low as 0.03 to 0.06% and consequently a sufficient amount of carbide (TiC) contributing to precipitation strengthening cannot be precipitated. As a result, the tensile strength of the obtainable steel sheets is only about 650 MPa. Increasing the Ti content in an attempt to increase the strength tends to result in the coarsening of TiC precipitated, and thus high strength exceeding 650 MPa cannot be easily achieved. In the technique proposed in JP '690, the carbide (TiC) is prone to be coarsened when the Ti content exceeds 0.06%. Thus, it is extremely difficult with that technique to increase the tensile strength of steel sheets to 850 MPa or more.
The technique proposed in JP '992 increases the strength of steel sheets by adding boron to the steel sheets as well as by adding manganese which is a solid solution strengthening element and is also an element that controls the precipitation of carbide contributing to precipitation strengthening. However, this technique involves addition of at least 10 ppm boron as illustrated in the Examples. Since boron significantly inhibits austenite recrystallization as will be described later, that proposed technique involving as much as 10 ppm or more boron cannot avoid problems associated with the anisotropy in mechanical properties. While JP '992 discloses an Example in which a steel sheet contains 0.5% manganese, the tensile strength of this steel sheet is as low as less than 750 MPa. Further, the anisotropy in mechanical properties of that hot rolled steel sheet will be large because it contains 0.03% niobium which significantly retards austenite recrystallization. Further, the technique proposed in JP '992 specifies the chemical composition of the steel sheet such that the ratio of the C content to the Ti content is inappropriate and does not allow the steel sheet to achieve a tensile strength of 850 MPa or more.
The technique proposed in JP '595 increases the strength of steel sheets by addition of manganese which is a solid solution strengthening element as well as promotes transformation and affects grain boundary shapes. However, the technique proposed in JP '595 is insufficient in terms of the optimization of rolling conditions and, as illustrated in the Examples, only provides steel sheets with a tensile strength of less than 850 MPa even when solid solution strengthening elements silicon and manganese are added in 0.5% and 1.5%, respectively. Further, the problematic anisotropy in mechanical properties is encountered. While JP '595 discloses an Example in which a steel sheet contains 0.3% manganese, the tensile strength of this steel sheet is as low as 730 MPa. Further, the anisotropy in mechanical properties of that hot rolled steel sheet will be large because it contains as much as 0.24% niobium which significantly retards austenite recrystallization. Furthermore, similarly to the technique proposed in JP '992, the chemical composition of the steel sheet is specified such that the ratio of the C content to the Ti content is inappropriate and does not allow the steel sheet to achieve a tensile strength of 850 MPa or more.
The technique proposed in JP '349, as illustrated in the Examples, only provides hot rolled steel strips with a tensile strength of about 70 kgf/mm2 when the Ti content is relatively low. Even when the ratio of the C content to the Ti content is appropriate, the tensile strength of the obtained hot rolled steel strips is still less than 850 MPa due to neglecting the austenite grain configurations. Further, JP '349 describes that the microstructure of the hot rolled steel strip is based on fine polygonal ferrite and homogenized to eliminate anisotropy. However, JP '690, JP '992, JP '595 and JP '349 does not disclose any specific parameters of the microstructure such as crystal grain diameters, and neglects the state of austenite grains. Thus, problems remain in the anisotropy of mechanical properties.
As discussed above, it is extremely difficult with conventional techniques to increase the tensile strength of hot rolled steel sheets to 850 MPa or more and simultaneously to avoid problems associated with the anisotropy in mechanical properties.
It could therefore be helpful to provide high strength hot rolled steel sheets having a tensile strength of not less than 850 MPa and a reduced anisotropy in mechanical properties evaluated in terms of tensile strength and total elongation, and to provide methods of manufacturing such steel sheets.